The present disclosure is directed to a telemetry network and more particularly to a system used with a spread of displacement senors making up a multistation set of equipment and thereby having multiple data generators. In greater detail, this disclosure is a method and apparatus for obtaining data from multiple sensors spread along a well. It finds application both in sensors placed in a well to record VSP data and to also obtain crosswell survey data.
One known logging technique is generally identified as VSP logging and involves a process whereby a sound source such as a vibrator or explosive charge is operated from the surface, sonic shock waves are then transmitted from the surface through multiple layers of the earth's crust, and the shock waves are reflected at various interfaces. A sensor is positioned in a well near the shock source. The sonic signals travel downwardly and reflect back toward the surface where they are intercepted and data is recorded from various geophone locations along the well. A crosswell survey involves positioning a sound source in an adjacent well, perhaps spaced apart by a few hundred feet, or even a few thousand feet from a well in which a sensor is positioned.
The present disclosure is directed to a system which reduces the time necessary to obtain data. More data is obtained in a shorter interval through the present approach. This involves positioning sets of displacement transducers at selected depths. In response to a single stimulus (which can be either an explosive, or a sweep sequence of a sound generator), data can be obtained at multiple locations. One of the difficulties with this, however, is that so much data is created that it is difficult to get all of the data out of the well and to the surface. In light of that, the present disclosure sets forth a telemetry system which holds some of the data and makes delayed transmission of that data to the surface.
The present disclosure sets out clusters of displacement transducers which are arranged in mutually orthogonal directions to provide three coordinate resolution of the sonic signal of interest. This is placed in a housing supporting the three sensors for installation at a particular depth. This is replicated at selected spacings along the well, for instance separate recording stations at a spacing of twenty-five feet. If only two or three sets of equipment are included, the data obtained from simultaneous operation is substantial, but it might possibly be transmitted to the surface through a conventional telemetry system; however, when the number of recording stations increases, data recovery is markedly increased far beyond what can be handled in logging cable telemetry. The present system sets forth a telemetry mechanism which enables recovery of the data. This also provides a common bus system from the stations which are deployed or spread along the well to transmit the data from the respective sensors to a main telemetry unit, and thereafter enables transmission to the surface. At the completion of each recording cycle, the telemetry system in the main unit will interrogate each of the appended seismic recording stations and obtain the necessary data. This is sequentially carried out until all the data has been transmitted to a memory in the main unit. In an overlapping fashion or at the completion of each sonic shock wave, data is transmitted from the main unit by telemetry to the surface. This latter transmission is particularly the more difficult transmission because it normally requires transmission over a much longer cable and it is limited to a maximum data rate.
This system is particularly advantageous over competitive devices. Heretofore, twisted pairs of wires have been used to deliver data to the telemetry unit. By contrast, it is much easier to provide a localized digitizing system and make a data transfer of digital data from subordinated telemetry units to a main telemetry unit for temporary storage in memory in a larger memory, and thereafter obtain transfer from the larger memory. Accordingly, each 3-axis recording station is provided with its own self-contained data acquisition system. Each one is thus provided with an amplifier, a filter, and an analog to digital converter (ADC) which are operated under control of a central processing unit (CPU) so that data is written into a memory in an organized fashion. That memory associated with that recording station is periodically interrogated and transmission is made by the station telemetry to the main unit which is provided with a separate telemetry system and main memory. The data is temporarily written in the main memory and is transmitted by the main telemetry system through the logging cable. As will be understood, the logging cable can be several thousand feet in length, and this great length provides some limitation on the data transfer rate appropriate for operation.
The present apparatus is summarized as a replicated seismic recording station spread comprised of M (a whole number integer) stations which are identical in construction. Each station is preferably provided with multiple listening devices, typically three orthogonal displacement transducers and an omnidirectional hydrophone (pressure transducer). In addition, a gyro can be provided which provides an indication of the angular position in space of the recording station. All of this structure is installed in a single housing along with a locking arm which locks the device rigidly against the sidewall of the well borehole. When it is locked in position, it is able to receive seismic shock waves from a remote explosive or tone source generator which are transmitted through the various formations with appropriate reflections and refractions thereby providing useful data. The analog signals output by the pressure and displacement transducers in the structure are appropriately amplified, filtered, converted to digital values, all in a timed sequence and recorded in localized memory. Thereafter, a telemetry unit transmits from the localized memory to a main memory and the main memory, in conjunction with a main telemetry system, transmits data up the logging cable for recovery at the surface.